In recent years, with the marked progress of information network technology and the expansion of services utilizing the information network, it is required for an electronic device to process a large amount of information and provide a high processing speed. In order to transmit a large volume of digital signals at a high speed, a signal with a shorter wavelength is effective, and the signal is evolving to a higher frequency signal. PPE has excellent high-frequency characteristics (i.e., dielectric characteristics), such as dielectric constant and dielectric loss tangent, as well as high heat resistance and therefore, is suitable as an insulating material for an electronic circuit board of an electronic device utilizing a high frequency band.
In general, PPE is originally a thermoplastic resin and therefore, is used together with a crosslinking curable compound at the time of use as an insulating material for an electronic circuit board. A cured product of a resin composition containing PPE and a crosslinking curable compound can have, as an insulating material for an electronic circuit board, both dielectric characteristics and heat resistance necessary for process compatibility to lead-free solder mounting, etc.
Many attempts have been made to provide a cured product of a resin composition containing PPE and a crosslinking curable compound and are described, for example, in Patent Documents 1 to 5 and Non-Patent Documents 1 and 2.
The resin composition containing PPE and a crosslinking curable compound, reported in Non-Patent Document 1, Patent Documents 1 and 2, etc., is characterized by a uniform structure with no phase separation between PPE and the crosslinking curable compound.
In Non-Patent Document 1, it is stated that PPE and a crosslinking curable compound have good compatibility and the cured product with the phase structure being a uniform phase is excellent in the heat resistance and is suitable as a low-dielectric/highly heat-resistant multilayered printed wiring board material. On the contrary, Non-Patent Document 1 indicates that when a phase separation structure of PPE and the crosslinking curable compound is observed, the adherence at the interface is reduced to deteriorate the heat resistance. Based on FIG. 2 of Non-Patent Document 1, the size of dispersed phase, at which the deterioration of heat resistance occurs, is from about 1.0 μm to about 17 μm.
Patent Document 1 describes a resin composition for a printed wiring board, containing PPE, an epoxy resin (crosslinking curable compound), and a curing agent for the epoxy resin, and it is stated that the composition does not cause a phase separation in the curing process and therefore, ensures high adhesiveness or heat resistance and excellent electric characteristics.
In Patent Document 2, it is stated that PPE and a thermosetting resin (crosslinking curable compound) exhibit a uniform morphology and a printed wiring board excellent in the heat resistance and electric characteristics can be thereby obtained. Patent Document 2 indicates that when an epoxy resin phase having a particle diameter of 1 μm or more is observed, the interface adherence between two phases is weak and the heat resistance is disadvantageously deteriorated.
On the other hand, Non-Patent Document 2 describes a structure where phase separation of PPE and a crosslinking curable compound is produced. In Non-Patent Document 2, it is stated that a thermally cured product of an extrusion molded article of a resin composition containing PPE, TAIC (crosslinking curable compound) and an initiator (α,α′-bis(tert-butylperoxy-m-isopropyl)benzene)) has a periodic structure.
In Non-Patent Document 2, application of the thermally cured product to a printed wiring board is not described, but in Patent Documents 3 and 4, it is stated that a cured product of a resin composition equivalent to the resin composition described in Non-Patent Document 2 is suitable for a printed wiring board. The resin composition described in Patent Document 3 contains PPE, TAIC and an initiator (Perbutyl P, α,α′-bis(tert-butylperoxy-m-isopropyl)benzene). The resin composition described in Patent Document 4 contains a styrene-butadiene copolymer, in addition to PPE, TAIC and an initiator (α,α′ bis(tert-butylperoxy-m-isopropyl)benzene). Patent Documents 3 and 4 are silent regarding the morphology of the cured product, but because of having a resin structure equivalent to that in Non-Patent Document 2, the morphology is also expected to be equivalent to that in Non-Patent Document 2.
In Patent Document 5, it is also stated that a resin composition containing PPE, an epoxy resin (as a crosslinking curable compound, a bisphenol A epoxy resin and a cresol-novolak epoxy resin), and an epoxy group-containing [styrene-butadiene-styrene] block copolymer takes on a phase separation structure.